Antennas are used to radiate and receive radio frequency signals. The transmission and reception of radio frequency signals is useful in a broad range of activities. For instance, radio wave communication systems are desirable where communications are transmitted over large distances. In addition, radio frequency signals can be used in connection with obtaining geographic position information.
In order to provide desired gain and directional characteristics, the dimensions and geometry of an antenna are typically such that the antenna is useful only within a relatively narrow band of frequencies. It is often desirable to provide an antenna capable of operating at more than one range of frequencies. However, such broadband antennas typically have less desirable gain characteristics than antennas that are designed solely for use at a narrow band of frequencies. Therefore, in order to provide acceptable gain at a variety of frequency bands, devices have been provided with multiple antennas. Although such an approach is capable of providing high gain at multiple frequencies, the provision of multiple antennas requires relatively large amounts of physical space.
An example of a device in which relatively high levels of gain at multiple frequencies and a small antenna area are desirable are wireless telephones capable of operating in connection with different wireless communication technologies. In particular, it may be desirable to provide a wireless telephone capable of operating in connection with different wireless systems having different frequencies, when communication using a preferred system is not available. Furthermore, in wireless telephones, a typical requirement is that the telephone provide high gain, in order to allow the physical size and power consumption requirements of the telephone components to be small.
Another example of a device in which high gain characteristics at multiple frequencies and a small antenna area are desirable are global positioning system (GPS) receivers. In particular, GPS receivers using dual frequency technologies, or using differential GPS techniques, must be capable of receiving weak signals transmitted on two different carrier signals. As in the example of wireless telephones, it is generally desirable to provide GPS receivers that are physically small, and that have relatively low power consumption requirements.
Still another example of a device in which a relatively high gain at multiple frequency bands is desirable is in connection with a communications satellite or a global positioning system satellite. In such applications, it can be advantageous to provide phased array antennas capable of providing multiple operating frequencies and of directing their beam towards a particular area of the Earth. In addition, it can be advantageous to provide such capabilities in a minimal area, to avoid the need for large and complex radiator structures.
Planar microstrip antennas have been utilized in connection with various devices. However, providing multiple frequency capabilities typically requires that the area devoted to the antenna double (i.e., two separate antennas must be provided) as compared to a single frequency antenna. Alternatively, microstrip antenna elements optimized for operation at a first frequency have been positioned in a plane overlaying a plane containing microstrip antenna elements adapted for operation at a second frequency. Although such devices are capable of providing multiple frequency capabilities, they require relatively large surfaces or volumes, and are therefore disadvantageous when used in connection with portable devices. In addition, such arrangements can be expensive to manufacture, and can have undesirable interference and gain characteristics.
The amount of space required by an antenna is particularly apparent in connection with phased array antennas. Phased array antennas typically include a number of radiator elements arrayed in a plane. The elements can be provided with differentially delayed versions of a signal, to steer the beam of the antenna. The steering, or scanning, of an antenna's beam is useful in applications in which it is desirable to point the beam of the antenna in a particular direction, such as where a radio communications link is established between two points, or where information regarding the direction of a target object is desired. The elements comprising phased array antennas usually must be spread over a relatively large area. Furthermore, in order to provide phased array antennas capable of operating at two different frequency bands, two separate arrays must be provided. Therefore, a conventional phased array antenna for operation at two different frequency bands can require twice the area of a single frequency band array antenna, and the phase centers of the separate arrays are not co-located. Alternatively, arrays can be stacked one on top of the other, however this approach results in antennas that are difficult to design such that they operate efficiently, and are expensive to manufacture. In addition, prior attempts at providing antenna arrays capable of operating at two distinct frequency bands have resulted in poor performance, including the creation of grating lobes, large amounts of coupling, large losses, and have required relatively large areas.
Therefore, there is a need for an antenna capable of operating at multiple frequencies that is relatively compact and that occupies a relatively small surface area. In addition, there is a need for such an antenna capable of providing a beam having high gain at multiple frequencies that can be scanned. Moreover, there is a need for an antenna capable of providing high gain at multiple frequencies that can be packaged within a relatively small area or volume, and that minimizes coupling and losses due to the close proximity of the antenna elements. Furthermore, it would be advantageous to provide an antenna capable of operating at multiple frequency bands and having co-located phase centers. In addition, such an antenna should be reliable and inexpensive to manufacture.